[VE][108] Line 8, Column 39: there is no attribute "async" from Graham Hawker on 2019-07-14 (www-validator@w3.org from July 2019)

From: Graham Hawker <ggh@connectedglobe.com>
Date: Sun, 14 Jul 2019 19:37:46 +0200
To: www-validator@w3.org
Message-ID: <f602ca74-8a7a-2d38-1c1f-e8b54ba7e6a4@connectedglobe.com>

I am receiving this error:
Validating https://metrotel.co.uk/jvt/
Error [108]: "/Line 8, Column 39/: there is no attribute "async""

However, on page https://www.w3schools.com/tags/att_script_async.asp it 
states clearly:
*In XHTML, attribute minimization is forbidden, and the async attribute 
must be defined as <script async="async">.*

So why is the validator giving me this error? The document starts with:
*
**<!DOCTYPE html      PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">**
**<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="es" lang="es">*

Received on Monday, 15 July 2019 06:57:42 UTC